It lacks EFI64 for arbitrary reasons. It's perfectly capable of having one. Apple just wants people to upgrade and buy new hardware.
The reason is thus:
it is 6 years old, been out of warranty/extended support for 3 years, and the EFI was never shipped back then as 64 bit.
Maybe it is technically capable of 64 bit, maybe not. Only apple really knows, and at the end of the day APPLE are the ones who are going to be dealing with customer issues trying to make ML run on antiquated hardware that is well out of support.
A decision has been made that the hardware is not supportable (on 6 year old hardware there are no guarantees that the hardware is not going to start showing defects when reinstalling the OS or installing an OS that makes use of features the original spec can't support), so they have dropped support.
There's nothing arbitrary about it - your hardware is very old and way out of support.
It sucks, but it happens to everyone's hardware eventually. Plan on turning your machines over every 3-5 years, tops, and you're not going to get caught out next time. If you can't afford a new mac pro that often, perhaps upgrading a lesser model more often will be more economically viable (and will likely give you better performance long term, anyhow)?
If you can get it working and "prove apple wrong" or whatever, go for it. Its likely not a case of being technically impossible anyway - simply that your machine is old and apple are no longer writing software/firmware for it. Try getting driver updates for 2006 spec PC hardware for Windows 8 and see how far you get.
Wasting programmer resources on supporting antiquated old hardware is only going to hold apple back from moving forward. Thus, they put a line in the sand, and anything without X as shipped is unsupported.
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